In hard disk drives used as mass storage devices within computing systems, one important limitation of increasing track density is non-repetitive runout of ball bearing assemblies within a disk spindle. The highly non-repetitive runout of a ball bearing system is thought to result from bearing geometry defects that cannot be attenuated by the very thin lubricant film between the rolling ball elements and the raceways of each ball bearing unit. A disk drive having a spindle with high runout results in a limitation upon the number of concentric data tracks that can be provided on the storage surfaces of the disk, because the tracks must be spaced sufficiently apart to accommodate the non-repeatable runout tolerances.
Hydrodynamic bearing systems provide one solution to this problem in that the spindle is running on a fully developed lubricant film that prevents contact between a shaft and a sleeve enveloping the shaft. Because of limited space available in contemporary small-form-factor disk drives, and because of a need to minimize prime costs, it is preferable to have a self-contained hydrodynamic bearing system with no external lubricant supply. For such bearing systems, preventing lubricant leakage has become a prime issue of concern.
Lubricant supply to hydrodynamic bearings is conventionally established by centrifugal force causing pumping of a lubricating liquid into a journal to form a substantially uniform bearing film between two relatively rotating members, such as a shaft and a sleeve or housing. It is also conventional to establish the pumping action of the bearing by defining relief grooves or a helical groove inclined at a specified angle relative to an axis or plane of rotation (as in the case of hydrodynamic thrust bearings) in one of the surfaces of the hydrodynamic bearing journal, the other surface being extremely smooth. Ideally, unidirectional relative rotation between the shaft and the sleeve causes the lubricating liquid to be pumped into the journal and maintained as a desired lubricant film layer therein under pressure.
Computer disk drives that use hydrodynamic bearings within disk spindle assemblies have commonly utilized one type of fluid bearing design, known in the art as a "herringbone" pattern bearing, or simply a "herringbone bearing". This label may be attributed to a repeating, generally symmetrical pattern of Vee-shaped relief grooves formed in either a shaft or in a bearing sleeve or housing. The ungrooved element has a smooth surface. Relative unidirectional rotation of the shaft and the sleeve causes the lubricating liquid to enter the legs of each Vee groove and flow toward an apex thereof, where fluid pressure from the resultant pumping action creates and maintains a hydrodynamic bearing during the relative unidirectional rotation between the shaft and its associated housing.
There have been a number of prior approaches for providing seals for hydrodynamic bearing units. Static seals, such as O-rings, and dynamic clearance seals, such as surface tension or capillary seals, have been employed to seal hydrodynamic bearings.
One prior example is found in Hendler et al. U.S. Pat. No. 3,778,123 entitled: "Liquid Bearing Unit and Seal". In the Hendler et al. approach, a non-wettable liquid, such as mercury, is placed in an annular Vee-groove at an outside boundary of the hydrodynamic bearing system. In addition, a thin film of low vapor pressure vacuum pump oil is provided at an annular gap or space at the end of a journal member in order to retain the mercury seal. A pair of thin barrier films are also provided at the outer edge of the annular space to prevent the oil from spreading as a result of surface effects and/or centrifugal forces generated by relative rotation of the bearing system.
Another prior approach is found in Van Roemburg U.S. Pat. No. 4,596,474, entitled: "Bearing System Comprising Two Facing Hydrodynamic Bearings". In the Van Roemburg approach, two radial fluid bearings were separated by a central reservoir. Each bearing included a herringbone pattern, and the herringbone patterns were such that the outer legs of the Vee-grooves forming the herringbone pattern were longer than the inner legs. However, the system maintained balanced pressure. This arrangement built up a lubricating liquid pressure at the apex of each Vee-groove which was greater than a counter pressure built up by the inner legs and by helical feed grooves which feed lubricant from a central reservoir area. By providing this differential pressure arrangement it is said that the lubricant was not pumped out of the bearing system.
A further prior approach is described in Anderson et al. U.S. Pat. No. 4,726,693, entitled: "Precision Hydrodynamic Bearing". The Anderson et al. approach uses a plurality of seals formed along the bearing unit including spiral grooves as well as an upper surface tension or capillary seal and a lower surface tension or capillary seal. However, the very nature of the Anderson et al. approach suggested that it was not adapted to omnidirectional operation or resistance to shock or vibratory forces.
Another prior approach is described in Titcomb et al. U.S. Pat. No. 4,795,275 and divisional U.S. Pat. Nos. 5,067,528 and 5,112,142, entitled: "Hydrodynamic Bearing". In the prior approaches described in these patents, surface tension dynamic seals were provided between axially extending surfaces of a thrust plate and bearing sleeve (or between tapered bearing surfaces). Pressure equalization ports were required and extended between the dynamic seals and interior lubricant reservoirs (or interior dynamic seals) to balance the hydrodynamic pressures in the lubricant in order to prevent the lubricant from being pumped through one of the dynamic seals. A method for introducing lubricating liquid into the hydrodynamic bearing employing a vacuum chamber and ultrasound is also described.
A similar prior approach is described in Pan U.S. Pat. No. 5,246,294 entitled: "Flow-Regulating Hydrodynamic Bearing". In this approach a disk spindle employs oppositely facing conical hydrodynamic bearing surfaces and a series of chambers and passages and a gravitational valve are provided to permit pressure-equalized centrifugally pumped global circulation of lubricating liquid drawn from one or more large reservoir volumes. A leak-preventing capillary trap "of minimum continuous axial length" may be provided at a clearance seal for passive capture of wandering lubricant when the bearing unit is at rest.
Small form factor disk drives are used in unlimited applications and orientations. Consequently, a hydrodynamic bearing system for a disk spindle in such drives must also operate in all possible orientations, and to be able to withstand and sustain certain shock events and vibration levels. A hitherto unsolved need has remained for a hydrodynamic bearing system which is leak free irrespective of orientation, shock and vibration.